1. Field of the Invention
The invention relates to a gas combustion engine comprising a gas exhaust system, an air intake system, an engine block comprising a number of cylinders and pistons, which pistons can be translated in the cylinders, and a cylinder head which delimits the space in the cylinders at one extremity, whereby the volume of the space in a cylinder between the cylinder head and a piston can be varied between a minimal and a maximal value by means of the translatory motion of the piston, and the ratio of the said minimal and maximal values defines a compression ratio, and a combustion space is defined by the space in that state of the piston which corresponds to a minimal volume. The invention also relates to a method of manufacturing such a gas combustion engine.
2. Description of the Related Art
Trucks and buses in Europe are fitted standard with diesel engines, for economic reasons (power, lifetime, fuel consumption). An alternative for diesel engines is formed by gas combustion engines running on LPG or natural gas. Disadvantages of gas combustion engines, however, are that they have a lower power and a higher fuel consumption than diesel engines. In addition, gas combustion engines are more expensive than diesel engines. This is caused inner alia by the fact that gas combustion engines are not yet manufactured by mass production techniques, and by the fact that the trucks and buses are delivered standard from the factory with diesel engines, so that modification is required if such a vehicle is to be made suitable for incorporation of a gas combustion engine.
An increasingly important disadvantage of diesel engines, however, concerns their pollution of the environment with exhaust gases. Such pollution is very much greater in the case of diesel engines than in the case of gas combustion engines. Gas combustion engines are generally known to be relatively clean engines, which pollute the environment to only a slight extent with exhaust gases which are much cleaner than those produced by diesel engines. This disadvantage of diesel engines is of even greater significance in the case of transport in densely populated areas, particularly in cities. In such areas, the disadvantages of the said pollution are felt to a greater extent than outside cities.
In order to alleviate the problem with respect to pollution, ever-increasing numbers of vehicles for intended use in city traffic are being fitted with gas combustion engines. Such vehicles are predominantly new ones, which are used to replace older, written-off vehicles, and are provided with new gas combustion engines during production; alternatively, the new gas combustion engine in such vehicles is provided during a revision service, and is a complete replacement for a former written-off diesel engine in the vehicle. In both instances, special modifications to the vehicles are necessary before the gas combustion engines can be fitted.
However, since the economic lifetime of diesel engines is long, which, for financial reasons, tends to be a dissuading factor in possible early replacement of the diesel engines, a large percentage of all such vehicles will remain fitted with diesel engines in the coming years, even if all new vehicles and revised vehicles were to be fitted with gas combustion engines.
An object of the invention is the provision of a financially attractive solution to the problem of pollution hereabove elucidated. To this end, the gas combustion engine in accordance with the invention is characterised in that the gas combustion engine is a converted diesel engine, whereby the combustion space and the compression ratio are adapted to application of gas as a fuel, whereby the pistons have a cylindrical wall and a top surface, whereby a hollow is located in the top surface, which hollow forms part of the combustion space and has an approximately circle-cylindrical form and an approximately flat bottom and the hollow has a diameter and a depth, whereby the ratio of the diameter to the depth lies between 30:9 and 30:13. The diesel engine is preferably converted to a gas combustion engine during a revision service.
From WO 91/14086 the conversion from a diesel engine into a gas engine is known per se, but
without reducing the compression ratio; PA1 without controling and varying the combustion point between 12.degree. and 20.degree. before TDC (top dead center) and PA1 without controlling the gas/air ratio using lambda probes and an engine regulation unit.
Consequently this known engine has a short life expectation and a high pollution of the environment. The diesel engine is preferably converted to a gas combustion engine during a revision service. The diesel engine is not replaced by an other engine, so that no (or, at least, practically no) modifications need be made to the vehicle. Above all, most of the diesel engine remains suitable for use as a gas combustion engine without any modification, as a result of which the depreciation costs are affected to a minimal extent (or even not at all). The costs of such a conversion are only slightly higher than the costs of revision of the diesel engine, so that there are hardly any reasons for not converting the diesel engine to a gas combustion engine during a revision of the former. Above all, it has transpired that the power and lifetime of such gas combustion engines are hardly inferior to those of diesel engines.
In an embodiment of the gas combustion engine in accordance with the invention, the exhaust gas system and the air intake system are also adapted to application of gas as a fuel.
In a diesel engine, there is often a hollow in-the top surface of the piston, on the bottom of which hollow is located a pointed protrusion serving to atomise the diesel fuel sprayed thereupon. A further embodiment of the gas combustion engine in accordance with the invention is characterised in that the exhaust gas system and the air intake system are also adapted to application of gas as a fuel. In this manner, the combustion space is given a form which is advantageous to a gas combustion engine, whereby an homogeneous mixture of gas and air accumulates in the hollow during operation. In addition, the compression ratio is hereby lowered, which is necessary when using gas as a fuel.
The enlarged hollow has such a diameter and depth that the ratio of the diameter to the depth lies between 30:9 and 30:13. Such a ratio ensures good heat extraction from the combustion-heated surfaces of the wall and bottom of the hollow. If this ratio is far removed from the stated range, then that part of the piston located between the piston wall and the wall of the hollow is unable to sufficiently impart its heat to the rest of the piston, leading to damage to the piston, which is detrimental to the lifetime of the engine.
In a preferential embodiment, this ratio is equal to 30:11. In experiments, this ratio transpired to be optimal with regard to heat transfer.
A further embodiment of the gas combustion engine in accordance with the invention is characterised in that the wall of the piston is bevelled at the extremity near to the top surface, whereby the external diameter of the piston decreases from a first value, remote from the top surface, to a second value, at the position of the top surface. As a result of the above-elucidated heating of the walls of the hollow, arising from combustion of the fuel, that part of the piston located between the piston wall and the wall of the hollow expands. This expansion is larger in proximity to the top surface of the piston than in proximity to the bottom surface of the hollow, due to better heat extraction near the bottom. As a result hereof, the diameter of the piston increases in the direction of the top surface, causing the piston to press against the wall of the cylinder, as a consequence of which the piston wall eats into the cylinder wall, which can cause serious damage. However, as a result of the bevelling hereabove referred to, there is room between the piston wall and the cylinder wall, so that expansion of the piston wall will not cause the piston wall to touch the cylinder wall.
An advantageous embodiment is characterised in that a number of piston rings are located in the wall of the piston, and that the said bevelling extends as far as the top surface and begins at that piston ring which is located nearest to the top surface.
Yet another embodiment of the gas combustion engine in accordance with the invention is characterised in that the exhaust gas system comprises two exhaust branch-pieces, each of which is connected via a curved pipe to a separate catalyser, whereby the exhaust gas system is remotely located from an axial line of a cylinder such that the cylinder head is freely accessible, when viewed in a transverse cross-section of the engine block. As a result hereof, one has access to the intake valves, exhaust valves and spark-plugs present in the cylinder head, without having to remove the exhaust branch-pieces, or at least without being hindered by the presence of the exhaust gas system. As is well known, diesel engines are not fitted with catalysers. The most obvious means of incorporating a catalyser system would be to fit a single regulated catalyser in the existing exhaust gas system. However, an advantage of the current embodiment according to the invention is that, with two catalysers of standard format, the same effect can be achieved as with a single catalyser of larger format, and at reduced costs, since the cost of two standard-format catalysers is lower than that of the alternative catalyser of larger format. Moreover, as a result of splitting the exhaust gas system into two parts, the temperature is divided over both parts, so that less substantial temperature-peaks occur, which has an advantageous influence on the thermal loading of both the parts and the catalysers.
An embodiment associated herewith is characterised in that a lambda probe is located in each pipe for the purpose of providing information to an engine unit. As a result hereof, it is possible to obtain better regulation of the operation of the catalysers.
In addition to the advantages already cited, which apply equally to the method in accordance with the invention, an embodiment of the method in accordance with the invention is characterised in that the volume of the intake branch-piece is reduced, and that a gas carburettor is coupled to the intake branch-piece via a short pipe. During operation, the intake branch-piece contains a fuel-air mixture, containing the air sucked in by the piston motion and a quantity of fuel injected by the carburettor. As a result of a reduced volume in the intake branch-piece, as well as a small volume in the connection between the gas carburettor and the intake branch-piece, the consequences of a possible backfire (i.e. a spreading of flame from the cylinder to the intake branch-piece, via a valve opening) are reduced, due to the smaller quantity of mixture in the intake branch-piece. In addition, there is a quickened reaction of the engine to a change in the position of the accelerator pedal, since the supply of old mixture having a composition corresponding to a former position of the accelerator pedal is more quickly depleted, allowing the new mixture with the new composition to be obtained more quickly in the cylinders.
A further embodiment of the method in accordance with the invention is characterised in that atomiser openings in the cylinder head are enlarged in diameter, and that spark-plug mounting-bases are fitted in these openings. As a result hereof, standard spark-plugs can be employed, and the need for specially made spark-plugs (which would increase the costs of the conversion) is removed.